1. Field of the Invention
This invention relates generally to printed circuit boards, and more particularly to an improved printed circuit board having plated through holes wherein wiring patterns can be formed over the plated through holes to thereby increase the usable space for printed circuitry. In more specific aspects, the present invention provides a printed circuit board wherein one or more layers of insulating material are applied over the board. The circuit board has plated through holes and each layer of insulating material has formed circuit lines thereon and vias for connection to the next underlying layer of metalization.
2. Prior Art
Conventional printed circuit cards and boards utilize drilled and plated through holes for communication between opposite sides and intermediate layers of the board to the wiring formed on both sides of the board. Frequently, the plated through holes will also receive pins for the connection of various types of circuit components. It is usually required that the plated through holes be able to act as a soldering socket for a module and that requires that they be fairly large and that a solderable ring or land of metal be provided around the plated through holes on the surface of the circuit board so as to allow for soldered connections. This type of structure dictates that a large amount of card or board space cannot be used for point to point wiring since the wiring must be done in the spaces between the holes, leaving room between the individual wire lines and between the wire lines and the solder rings. Recently there has been introduced surface modules and thin film chip wiring devices that do not have pins but have other types of connection pads. With this type of structure the space necessary for the holes can be reduced somewhat; nevertheless even with this technology a significant amount of space is used by the holes which is not available for surface wiring. Various techniques have been suggested for various types of wire interconnection wherein plated through holes are present on the board. For example, U.S. Pat. No. 3,356,786 to Texas Instruments shows a technique where conducting lines can be provided which extend across various openings or holes. This technique, however, has many limitations both in the structures available and the utilization of the holes. Other patents that show techniques for increasing somewhat the wiring density, are U.S. Pat. Nos. 4,554,405 to International Business Machines Corp.; U.S. Pat. No. 4,581,679 to TRW, Inc.; U.S. Pat. No. 4,535,388 to International Business Machines Corp.; U.S. Pat. No. 4,598,166 to GTE Communication Systems; and U.S. Pat. No. 4,179,800 to DuPont Electric Co. None of these are completely effective to utilize the entire surface area of board.